The present invention relates to waveform shaping circuits, and more particularly to a waveform reshaping circuit for a digital signal which is transmitted (or recorded and reproduced).
For instance, in a digital audio system, a digital signal whose instantaneous level varies with sound data is recorded and reproduced. In the digital audio system, it is difficult to increase the bandwidth of the recording and reproducing system, and therefore the high frequency components of the recorded and reproduced digital signal are attenuated. Accordingly, the waveform of the digital signal is distorted, and its rise and fall times are made longer. Thus, the period of time that the instantaneous level of the recorded and reproduced digital signal is close to the threshold level is increased, increasing the probability of coding errors. Accordingly, it is essential for the digital audio system or the like to have a waveform reshaping circuit for reshaping the waveform of a distorted digital signal in order to correctly extract the data of the digital signal which is recorded and reproduced.
A conventional waveform reshaping circuit of this type is constructed as shown in FIG. 1. In this circuit, a transmitted (or recorded and reproduced) digital signal a is applied through an input terminal IN.sub.1 to the positive input terminal of a comparator 1. A voltage V.sub.0, which is obtained by dividing a supply voltage with resistors R.sub.1 and R.sub.2, is applied to the negative input terminal of the comparator 1. The output of the comparator 1 is raised to a high level when the instantaneous level of the digital signal a exceeds the voltage V.sub.0, and it is set to a low level when the instantaneous level is equal to or lower than the voltage V.sub.0. Thus, the comparator 1 provides a reshaped digital signal at an output terminal OUT in which the rise and fall times are made considerably short by waveform shaping.
On the other hand, if the waveform of a digital signal which corresponds to a bit string as shown in FIG. 2A and which is to be transmitted (or recorded and reproduced) has the actual distorted form shown in FIG. 2B because the bandwidth of the recording and reproducing system is not sufficiently wide, then coding errors occur, and thus the reshaped waveform of the distorted digital signal a supplied to the comparator 1 may be as shown in FIG. 2C. In the digital signal, because the rise and fall times are relatively long, the instantaneous level in the signal is affected by the code of the preceding bit. That is, in the waveform, it is rather difficult to discriminate the bit levels. For instance, when the two preceding bits are both "0", the instantaneous level for "1" becomes extremely low, and when the two preceding bits are both "1", the instantaneous level for "0" becomes extremely high. Accordingly, the comparator 1 outputs a digital signal, as shown in FIG. 2C, in which the duration of an interval corresponding to a bit varies. At a time instant in the duration of the interval, it is necessary to discriminate the code of the bit, thereby to extract the data. If the duration of the interval is made shorter, a jitter margin, i.e., a data extraction margin with respect to a time axis variation, is decreased. Therefore, the conventional waveform shaping circuit suffers from a difficulty that the probability for a coding error to occur is increased when coding interference occurs.